In a thermal power generation coal fired boiler apparatus for firing pulverized coal as fuel, a vertical roller mill is used in a fuel supply device. A conventional example thereof is shown in FIG. 27.
This vertical roller mill has a grinding zone 5 which pulverizes coal as a raw material of finely pulverized coal by inducing between a grinding table 2 and heavily loaded grinding rollers 3, and a classification zone 6 which is provided on top of the grinding zone 5 so as to classify pulverized coal into an arbitrary particle size.
Describing the operation of the vertical roller mill, a subject 50 of pulverization as coal fed from a coal supply pipe (raw material supply pipe) 1 falls down to a center zone of the rotating grinding table 2 and then moves to an outer circumferential zone thereof while tracing a vertical locus on the grinding table 2 based on centrifugal force caused by the rotation of the grinding table 2 as represented by arrows, so that the subject 50 is induced between the grinding table 2 and the grinding roller 3 and pulverized.
The pulverized subject is blown up while dried by hot air 51 introduced from a throat 4 provided in the circumference of the grinding table 2. Part of the blown-up powder large in particle size falls down 55 by gravitation during conveyance toward the classification zone 6 so as to return to the grinding zone 5 (primary classification).
The group of particles which have reached the classification zone 6 are classified into fine particles 54 smaller than a predetermined particle size and coarse particles 53 not smaller than the predetermined particle size by the classification zone 6 (secondary classification). The coarse particles 53 fall down to the grinding zone 5 located in a lower zone of the vertical pulverizing apparatus, so that the coarse particles 53 are pulverized again. On the other hand, the fine particles 54 which have come out of the classification zone 6 are fed to a boiler (not shown) through a coal feed pipe (product fine powder discharge pipe) 30.
As shown in FIGS. 28 and 29, a two-stage type classification device composed of a combination of a stationary classifier 10 disposed in an inlet of the classification device and a rotary classifier 20 disposed in the inside of the stationary classifier 10 is generally used as the conventional classification device forming the aforementioned classification zone 5.
The stationary classifier 10 is hung down from a classification zone top plate 40, and has a large number of stationary fins 12 which are arranged in a circumferential direction and which are disposed at an arbitrary angle with respect to the center axis direction of the classification device, and a rectifying cone 11 which is shaped like a downward convexly conical shape and which is provided under the stationary fins 12. The rotary classifier 20 has a large number of rotary fins 21 which are provided in a circumferential direction and which are disposed at an arbitrary angle with respect to the center axis direction of the classification device so that a length direction of a plate faces a vertical direction.
The operation of the two-stage type classification device will be described with reference to FIGS. 28 and 29. A solid-gas two-phase flow 52 blown up for below and introduced into the classification device is rectified and at the same time subjected to weak swirling in advance when the solid-gas two-phase flow 52 passes through the stationary fins 12.
When the solid-gas two-phase flow 52 has reached the rotary fins 21 rotating at a predetermined rotational speed on the center axis of the device as an axial center, strong swirling is given to the solid-gas two-phase flow 52 so that force to bounce out particles in the solid-gas two-phase flow 52 to the outside of the rotary fins 21 by centrifugal force is applied on the particles in the solid-gas two-phase flow 52. On this occasion, centrifugal force applied on coarse particles 53 with large mass is large, so that the coarse particles 53 are separated by an air flow passing through the rotary fins 21. The coarse particles 53 settle down sedimentarily in a space between the rotary fins 21 and the stationary fins 12 by gravitation, so that the coarse particles 53 finally fall down to the grinding zone 5 provided as a lower zone, along an inner wall of the rectifying cone 11.
On the other hand, fine particles 54 are carried with the air flow because centrifugal force applied on the fine particles 54 is small, so that the fine particles 54 pass through the rotary fins 21 so as to be discharged to the outside of the vertical pulverizing apparatus as shown in FIG. 27. Incidentally, the particle size distribution in product fine powder can be controlled when the number of revolutions of the rotary classifier 20 is adjusted. Incidentally, a reference numeral 22 in the drawing designates a direction of rotation of each rotary fin 21, and a reference numeral 41 designates a classification portion outer circumference housing.
FIG. 32 is a schematic configuration diagram showing the whole of a coal fired boiler apparatus having this vertical roller mill. Combustion air A fed in by a forced draft fan 57 branches out into primary air A1 and secondary air A2. The primary air A1 branches out into air directly fed as cold air to the vertical roller mill 59 by a primary air forced draft fan 58 and air fed to the vertical roller mill 59 after heated by an exhaust gas type air preheater 64. Then, the cold air and warm air are mixed and adjusted to optimize the temperature of mixture air, so that the mixture air is fed as the hot air 51 to the vertical roller mill 59.
After raw coal which is a subject 50 of pulverization is put into a coal banker 65, a predetermined quantity of the raw coal is fed to the vertical roller mill 59 by a coal feeder 66 and pulverized. The generated finely-pulverized coal pulverized while dried with the primary air A1 is conveyed with the primary air A1 and fed to a boiler 67 through a pulverized coal burner in a wind box 68, so that the pulverized coal is ignited and burned. The secondary air A2 is heated by a steam type air preheater 69 and the exhaust gas type air preheater 64 and then fed to the wind box 68, so that the secondary air A2 is provided for burning of pulverized coal in the boiler 67.
There is provided a system in which an exhaust gas produced by firing of pulverized coal is discharged from a stack 74 to the atmospheric air after dust is removed by a dust collector 70, nitrogen oxide (NOx) is reduced by a denitrater 71, the exhaust gas is sucked in by a induced draft fan 72 via the exhaust gas type air preheater 64 and a sulfur component is removed by a desulfurizer 73.
For example, the following Patent Document concerned with the classification device can be listed below.    Patent Document 1: JP-A-2002-233825